The project's aim is to identify key intracellular mechanisms underlying gonadal steroid regulation of gonadotropin secretion. Radioimmunoassay and neurochemical techniques have been employed to monitor patterns of gonadal steroid nuclear uptake in brain and pituitary tissues that are associated with changes in pituitary responsiveness to GnRH resulting from steroid exposure. Timed studies on the influence of the antiestrogen CI-628 on 3H- estradiol uptake suggest that pituitary gland uptake of estradiol is crucial for the expression of rapid feedback action of this steroid to suppress tonic LH release. This conclusion has been confirmed in a dispersed pituitary cell perifusion system in vitro. Using an estradiol "exchange assay" to measure estrogen-receptor complexes in nuclear pellets of brain and pituitary tissues we have characterized nuclear estradiol receptor occupation throughout the rat estrous cycle and have developed estradiol treatments which can replicate these occupation levels in castrate animals. We are currently examining the ability of these estrogen treatments to stimulate cyclic gonadotropin secretion. The exchange assay has also been used to show that androgen regulation of tonic gonadotropin secretion is mediated by androgen receptors and not by aromatization of androgen to estrogen. In contrast, androgen induction of cyclic LH release does appear to depend on testosterone-estradiol conversion within the brain. Finally, studies utilizing an exchange of pituitary glands between pseudohermaphrodite and normal littermate rates have been attempted to localize the site of androgen feedback control of LH release to the brain or the pituitary gland.